1. Field of the Invention
The present invention relates to filters for reducing unwanted electrical noise within audio reproduction equipment and more particularly, to power line noise filters for use in conjunction with audio reproduction equipment.
2. Description of the Prior Art
The audio reproduction quality of electronic audio equipment is often degraded by the effects of noise in the power line feeding the equipment. If the power is supplied in an automotive environment, there is often a significant level of line noise. This noise can originate from a number of sources within a vehicle's electrical power system, the most common source being the vehicle's alternator, which may cause electrical noise from the brush contacts and will cause power line ripple due to the A.C. to D.C. conversion step inherent in alternator designs. This power line noise and/or ripple can manifest itself as a whistle, a whine or a hiss at the output of audio reproduction equipment connected to the power source. The frequency of the noise and therefore of the resultant audio disturbance will often vary with engine speed and significantly degrades the quality of the perceived audio output.
Heretofore, many attempts have been made to reduce the effects of line noise on audio reproduction characteristics. Commonly, resistive, inductive and capacitive elements are used to construct power line filters. These devices or methods have been at least partially satisfactory in the past in reducing line noise to provide acceptable audio reproduction characteristics. However, in order to filter out relatively low frequency alternator ripple, large inductors and large capacitors are typically required, typically resulting in high power loss. Further, the components are costly and require considerable space, making them incompatible with various integrated component fabrication techniques. Moreover, certain equipment, such as high power audio amplifiers which create high current drains, will require even larger scale filter components which require considerable space and create correspondingly greater losses and added expense.
Many automotive stereo amplifiers, for example, draw in excessive of 20 amps. At currents in this range, a tenth of an ohm of resistance in a filter coil will result in a two volt drop in the power supply line voltage. Since power loss is dependent upon the square of the voltage loss, a two volt drop in a typical twelve volt system will result in a 31% power reduction. The seriousness of the resultant power loss in terms of reduced performance and audio fidelity is readily apparent.
In addition to the need for effective, efficient power line filtering, audio amplification equipment must be provided with circuitry for switching the equipment off and on. When amplification is desired, it becomes necessary to switch the amplification means on to provide appropriate audio signal level enhancement. After amplification is no longer necessary or desired, the amplification must be switched off. In present automotive audio equipment, a low power, low level signal typically is produced from a microprocessor or light duty switch within the radio to signal the requirement for audio amplification. This low level control signal is received by the amplifier, and the switching is often implemented by the use of a relay. However, relays can add unnecessary expense, size, weight, and power consumption to an audio system. With present electronic packaging which is optimized for light weight and compact size, more efficient switch circuitry is preferable.